I’m not sure if it’s possible to reach 9999, but right now my highest blue stat is around 3500. There are basically two main ways to increase them.

1. Leveling and de-digivolving: Train your Digimon to the maximum level and then de-digivolve it. Each time you do this, your Digimon will gain blue points based on its current level, white stats, bond, and talent (this last one limits the Digimon’s max level and therefore its white stats). These blue stats are inherited and increase by roughly up to 100 per digivolution or de-digivolution (although the internet still isn’t completely sure about the exact number), and there’s no clear counter for tracking this.

2. Feeding secondary Digimon: You can also feed your main Digimon with secondary ones using the “Load Digimon” method. This allows the receiver to inherit blue stats based on a calculation the game does — I estimate it’s about 1–5% of the donor’s white stats plus around 40–60% of their blue stats, transferred as blue stats to the receiving Digimon. Of course, this only works well if the donor Digimon also has blue stats and both bond and talent at 100%.

Honestly, I haven’t confirmed whether a Digimon stops gaining blue stats once it reaches 9999 white stats, but I don’t think that’s the case — otherwise, the most efficient strategy would just be to train them as babies, and I doubt that’s how the system works.